If i understand you right, you need to get alerted before this happens? if so you may achieve this by:
1) Using Events
You can set a global event from system admin or per domain from domain admin to send you or/and user or and domain admin a message
Create Additional Disk Usage Event (before disk is full ) :
=========================================
Category : User
Event Type : User Disk Space Used
Mailbox Usage (%) : choose Greater than from the menu box and enter 90 for example
Go to Actions button and add action:
Action : Send mail
From : " #domain# System Admin"<admin@#domain#>
To Address : #emailaddress# (or both) admin@#domain#; #emailaddress#
Body :
Dear #emailaddress#,
Your mailbox at #emailaddress# is now #mailboxusagepercent#% full (approximately #mailboxusagemb#MB).
Allowed size for this mailbox is #mailboxsizemax#MB.
----------------------------------------------------------
#domain# System Admin
Click save and remember to click save Event on top
2) Content Filtering
Ad filter to copy message or forward it to you when has subject "Mailbox size limit exceeded" (if the default message) and might add "has been exceeded" in body
- Go to Domain Settings > Filtering > Content Filtering
- Click new and choose subject and body then add the above